Direct bonding

There is demand to lower the process temperature due to several factors, one is for instance the increasing number of utilized materials with different coefficients of thermal expansion.

[6] The surface state of a silicon wafer can be measured by the contact angle a drop of water forms.

Before bonding two wafers, those two solids need to be free of impurities that can base on particle, organic and/or ionic contamination.

The actual procedure needs to be adapted to every application and device because of usually existing interconnects and metallization systems on the wafer.

In consequence a significant fraction of Si-OH (silanol) groups start to polymerize at room temperature forming Si-O-Si and water and a sufficient bonding strength for handling the wafer stack is assured.

[8] After the pre-bonding in air, in a special gaseous atmosphere or vacuum, the wafers have to pass an annealing process for increasing the bonding strength.

The annealing therefore provides a certain amount of thermal energy which forces more silanol groups to react among each other and new, highly stable chemical bindings are formed.

The kind of binding which forms directly depends on the amount of energy which has been delivered or the applied temperature respectively.

At temperatures from 110 °C to 150 °C silanol groups polymerize to siloxane and water, but also a slow fracture takes place.

Above 800 °C native oxide gets viscous and starts to flow at the interface, which increases the area of contacted surfaces.

The bonding at room temperature is mostly based on van-der-Waals forces between those hydrogen and fluorine atoms.

This fact builds up the need for a higher surface quality and cleanliness to prevent unbonded areas and thereby to achieve a full-surface contact between the wafers (compare infrared photograph of a bond wave).

The difficulty is that water molecules may react with already formed siloxane-groups (Si-O-Si), so the overall energy of adhesion gets weaker.

[2] Lower temperatures are important for bonding pre-processed wafers or compound materials to avoid undesirable changes or decomposition.